1) Field of the Invention
The present invention relates a method and a system for controlling a shift of a transmission speed ratio for a gas turbine engine vehicle, by which enhanced response characteristics in a transition from steady state to acceleration state are provided.
2) Description of the Related Art
Due to the ease of operation thereof, a two-shaft type gas turbine engine is principally used as an automotive gas turbine engine. FIG. 1 shows a typical construction of a power train employed in a gas turbine engine vehicle having an automatic power transmission.
In such a two-shaft type gas turbine engine, the engine is started by driving a front gear F/G, and then intake air is compressed by a compressor C, heated by a heat exchanger HE, and combusted in a combustion chamber CC by mixing same with a fuel supplied from an actuator A1. The thus-generated combustion gas drives a compressor turbine CT arranged coaxially with the compressor C. The compressor turbine CT and the compressor C as combined will be referred to hereinafter as "the gas generator GG". The compression at the compressor C varies in accordance with the revolution speed of the compressor turbine CT. The combustion gas used for driving the compressor turbine CT is transferred through a variable nozzle VN, the combustion gas path area of which is adjustable by an actuator A2, for driving a power turbine PT, and the combustion gas then passes through the heat exchanger HE and is exhausted to the atmosphere as an exhaust gas.
The actuators A1 and A2 are controlled by a control circuit CONT, in accordance with the driving conditions of the engine. This control is enabled by an input of an accelerator pedal depression magnitude and engine driving parameters from sensors (not shown) to the control circuit CONT. In FIG. 1, encircled figures indicating the intake air pressure P and temperature T represent the intake air and temperature at the corresponding position thereof.
A revolution speed N2 of the power turbine PT is reduced to a rotation speed N3 by a reduction gear R/G, and the rotation speed N3 is then further adjusted by an automatic power transmission A/T, in accordance with a shift position thereof and the engine driving conditions. The output of the automatic power transmission is then transferred to a differential gear unit D via a propeller shaft, to drive wheels W mounted on a wheel axle, and thus drive the vehicle.
In general, the automotive automatic power transmission with a torque converter is controlled according to predetermined shift characteristics, in terms of an input torque and a vehicle speed. The vehicle speed is normally obtained by monitoring a rotation speed at an output shaft of the automatic power transmission A/T. In the case of a reciprocation type engine such as a gasoline engine or Diesel engine, however, the input torque is obtained from an accelerator pedal depression magnitude, which is proportional to the output torque of the engine. Conversely, in the case of a gas turbine engine, since there is a plurality of parameters affecting the input torque, it is not possible to detect the torque solely by the accelerator pedal depression magnitude. Therefore, in the prior art, the engine output torque is detected by utilizing parameters such as the output pressure P3 of the compressor, the operating position of the variable nozzle NV, and so forth, as disclosed in Japanese Unexamined Patent Publication (Kokai) 61-163030.
FIG. 2 shows one example of a shift pattern of the automatic power transmission, in which the abscissas represents the rotation speed at the output shaft of the automatic power transmission, and ordinates represent a displacement magnitude .theta.th of the throttle cable (accelerator pedal depression magnitude .theta.acc). In the prior art, a shift control for the automatic power transmission A/T utilizes a shift characteristics map representing the shift pattern as shown, in terms of the rotation speed Np (vehicle speed) of the output shaft of the automatic power transmission and a parameter corresponding to the engine output torque (in the shown example, the accelerator pedal depression magnitude .theta.acc).
When the automatic power transmission employed in an automotive vehicle having a two-shaft type gas turbine engine is controlled by utilizing the shift pattern of FIG. 2, which uses the accelerator pedal depression magnitude .theta.acc and the vehicle speed, the vehicular running performance is satisfactory. Particularly, in the two-shaft type gas turbine engine vehicle, a problem arises in that the acceleration response characteristic is low and causes an acceleration lag which has an adverse affect on the drivability during the transition from a steady state to an acceleration state.
This problem will be further discussed in detail. The output characteristic of the gas turbine engine is illustrated in FIG. 3, wherein the revolution speed N1 of the gas generator GG is a primary factor for determining the engine output. The rotation speed N3 at the engine output shaft obtained through the reduction gear R/G, however, is determined in accordance with selected speed ratio of the automatic power transmission which is associated with the torque converter, and the vehicle speed. The revolution speed N1 of the gas generator GG is determined to establish a balance between an engine output power demanded by the vehicle and the actual engine output, independently of the rotation speed N3 at the output shaft of the engine. Accordingly, when the two-shaft gas turbine engine is used as a prime mover of the automotive vehicle, it is important to obtain satisfactorily high response characteristics of the revolution speed N1 of the gas generator GG, which is a primary factor for determining the engine output, in the acceleration and deceleration states, to thus enhance the vehicular driving performance, since the fluctuation of the output at the automotive engine is substantially large.
Especially, in a conventional vehicle employing a gas turbine engine, and adjusting the driving torque delivery by adjusting the speed ratio through the automatic power transmission, it is general practice to set a target revolution speed N1set of the compressor turbine CT corresponding to the accelerator pedal depression magnitude, and to adjust a fuel flow rate Gf to supply fuel to the engine so that the target revolution speed N1set of the compressor turbine CT is reached. During this process, a lag occurs in the response during an adjustment of the revolution speed N1 of the compressor turbine CT versus the accelerator pedal depression magnitude .theta.acc. The magnitude of the response lag occurring in a normal gas turbine engine is over 1 sec. at 10000 r.p.m. Accordingly, during a transition from the steady state to an acceleration state, the acceleration of the vehicle is unacceptably slow, and during a decelerating transition, a sufficient deceleration cannot be obtained quickly enough to satisfy the required deceleration. Such low acceleration and deceleration characteristics will adversely affect the vehicular drivability.